1. Field of the Invention
The present invention relates to a reluctance motor, and particularly to a rotor structure thereof.
2. Description of the Background Art
Among motors presently used in many technical fields, there are motors that comprise a rotor formed of a material having high magnetic permeability rather than of a permanent magnet, and having a rotor cross-sectional structure such as shown in FIG. 15 wherein a plurality of nearly-insulated split magnetic paths are disposed between adjacent poles. Such motors are known as reluctance motors. As illustrated in FIG. 15, a rotor 2 of a typical reluctance motor is provided with a plurality of slits 3, in addition to a hole for receiving a motor shaft 5. These slits 3 serve as the basis for forming magnetic paths in the rotor 2. Specifically, as known in the conventional art, when electric current is made to flow in coils wound in slots 8 (FIG. 17) of a stator surrounding the rotor 2, magnetic flux is generated between the energized coils in the direction according to the right-hand rule. As the rotor 2 is formed of a material having high magnetic permeability, the magnetic flux passes through the rotor 2. Air is present in the slits 3, and, as known, magnetic permeability of air is extremely low compared with that of a high magnetic permeability material. When the slits 3 are formed in the rotor 2, magnetic flux passes via magnetic paths located between the slits 3 of the rotor 2 (FIG. 16). In light of the above, when, for example, electric current is made to flow in coils located in direction d indicated by dotted lines in FIG. 16, magnetic flux can be generated as illustrated by dot-and-segment arrows in FIG. 16 if the slits 3 are formed in the rotor 2.
When energizing the coils and forming desired magnetic flux as described above, it would be ideal if all magnetic flux from the N pole to the S pole is formed along the plurality of split magnetic paths between the slits 3, as shown in FIG. 16. However, in reality, this is not the case. Specifically, a current must be made to flow not just in the coils located in direction d in FIG. 16, but also in coils located in direction q to provide rotational force to the rotor 2. In practice, as known, a current formed by vector synthesis of these currents are made to flow in each of the coils. Accordingly, in such states as shown in FIG. 16, magnetic flux is generated in a peripheral portion 7 of the rotor 2 and in a direction perpendicular to the slits 3 in the rotor 2, for example, as illustrated in FIG. 17. These magnetic fluxes do not function to form magnetic poles in the rotor 2 but form magnetic flux leaks. In FIG. 17, although only two lines of magnetic flux leaks are indicated as typical examples, many magnetic flux leaks actually exist in various directions. When magnetic flux leaks are generated, the total magnetic flux may shift the N poles and S poles from the positions indicated in FIG. 16, causing the unfavorable decrease in generated torque.
Further, the following problem is present in the structure wherein each slit 3 is continuous from one end to the other. In such a structure, each split magnetic path of the rotor 2 is supported only at extremely thin portions indicated as the periphery portion 7 of the rotor. When the rotor 2 rotates, this thin portion may not be able to withstand the centrifugal force generated on the rotor 2, and may thus deform or break. It is therefore necessary to include measures against the centrifugal force when deciding the shape of the slits 3 in the rotor 2. One example of a conventionally proposed measures is the shape shown in FIG. 18. FIG. 18 illustrates a cross-sectional shape of a rotor similar to FIG. 15, but it differs from FIG. 15 in that the slits 3 are interrupted once along the halfway of the split magnetic paths, and portions connecting adjacent split magnetic paths are formed. The portions that mechanically link the split magnetic paths to one another as described above are referred hereinafter as connecting portions. As shown in FIG. 18, by including the connection portions 6, a strong structure is provided that can withstand the centrifugal force. Although a variety of other widths, shapes, locations, and numbers of connecting portions 6 are possible, the following explanation will be based on the example of FIG. 18.
Leaking magnetic flux is increased in the example of FIG. 18 because leaking magnetic flux generates, not just in the periphery portion 7 of the rotor 2 and in the direction perpendicular to the slits 3, but additionally along the connecting portions 6, as shown in FIG. 19. This situation results in further decrease in generated torque.
As generation of magnetic flux leaks result in decrease of output torque, it is most preferable if magnetic flux leaks are minimized. Accordingly, it is theoretically desirable that connecting portions 6 and the periphery portions 7 do not exist. However, as explained above, the connection portions 6 and the periphery portions 7 are mechanically required with respect to the centrifugal force generated on the rotor 2. These contradicting requirements must be somehow fulfilled. Conventionally, for example, a rotor structure as shown in FIG. 20 was devised wherein permanent magnets 4A are disposed near the periphery portion 7 of a rotor 2 without connecting portions 6. This structure was intended to eliminate leaking magnetic flux in connecting portions 6, and to reduce leaking magnetic flux in the periphery portion 7 through magnetic saturation provided by the permanent magnets 4A. Such an arrangement, however, presented more difficult conditions relating to centrifugal force because, further to the difficulty of withstanding centrifugal force only by the mechanical strength of the periphery portion 7, additional centrifugal force is generated by the permanent magnets 4A,.
One object of the present invention is to solve the problem of decrease in generated torque due to magnetic flux leaks at the connecting portions 6.
Another problem to be addressed is that, as the internal shape of the rotor is rather complex, rotor mechanical strength is insufficient. A new rotor structure is necessary which can withstand the centrifugal force generated on respective portions of the rotor and the reaction force of the torque generated by the reluctance motor.
A further problem exists when many expensive permanent magnets are used, as in the example of FIG. 20. Overall cost increases with the costs of permanent magnets and the costs for assembling those magnets.
Another problem is the unfavorable increase of torque ripples which result from disposing permanent magnets on the internal portions of the rotor or from taking measures to increase rotor mechanical strength.
A further problem is the insufficient rotor mechanical strength due to the many empty portions inside the rotor structure. Although the shapes shown in the above-mentioned Figures can be used for general applications, further strengthening of the rotor structure is necessary for particular applications wherein a strong impact may be applied.
The purpose of the present invention is to simultaneously solve the above problems and provide a practical reluctance motor. The required conditions of a practical motor for general applications are that sufficient motor torque can be obtained, rotational operation at high speeds is possible without any problems concerning rotor mechanical strength, motor cost is low, and torque ripple is minimized.